Fail-safe improvement for a flexible shaft coupling

ABSTRACT

A fail-safe flexible coupling is provided for coupling shafts, typically in an aircraft, wherein the coupling includes a plug coupled to the first shaft and a socket coupled to the first shaft and a socket coupled to the second shaft. The plug is arranged and configured to loosely mate with the socket when the shafts are being rotated together by virtue of their primary coupling. When the primary coupling fails, the socket and plug engage each other securely though the shafts continue to be driven notwithstanding the failure of the primary coupling between them. The plug is particularly characterized by a screw portion and head portion. The screw portion is threaded into the first shaft. The coupling is secured to the first shaft by the head portion of the plug.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of couplings between rotatingshafts and in particular relates to improvements in flexible couplingsbetween shafts whereby the shafts may continue to be driven even thoughthe flexible coupling means between the shafts fails.

2. Description of the Prior Art

Flexible couplings are used in mechanical applications wherever twoshafts need to be coupled and are not precisely coaxial at all times.One of the primary applications where this occurs is where there is asubstantial amount of vibration, such as in vehicles and, in particular,in aircraft.

In an aircraft application not only are vibrational stressesparticularly exaggerated, but all elements of the shaft and couplingmust be extremely light weight and yet capable of carrying extremelyhigh loads with a very high reliability. One of the flexible couplingsused throughout aircraft is a "Bendix coupling" manufactured by theBendix Corporation, one example which is illustrated in the drawings andwhich is sold and distributed as Bendix Model No. 19E111-1 and 19E149.

In the case where a flexible coupling is used to couple two shafts usedin the power train in a helicopter, it is extremely important that thecoupling has a high degree of reliable performance in a very adverseenvironment. Where the flexible coupling is used in the shaft whichdrives the tail rotor in a helicopter, it is important that power bedelivered to the tail rotor at all times without interruption. Shouldpower delivery to the tail rotor be lost, serious consequences may ariseby the unexpected loss of control.

Therefore, what is needed is a lightweight and extremely reliable meanswhereby an improvement can be made in a flexible coupling used betweentwo shafts whereby the shaft will be securely driven or engagednotwithstanding the failure of the coupling.

BRIEF SUMMARY OF THE INVENTION

The present invention is an improvement in a flexible coupling having afirst member coupled to a first shaft and a second member coupled to asecond shaft. The first and second members are coupled by means of aflexible portion of the coupling. The improvement comprises a plug meansfor coupling to the first shaft and a socket means for coupling to thesecond shaft. The plug means is arranged and configured to loosely matewith the socket means in a first condition, namely the condition inwhich the flexible coupling has not failed, and then to securely engagethe socket means in a second condition, namely the condition in whichthe flexible coupling has failed. By virtue of this improvement when theflexible coupling fails, the plug and socket means securely engage eachother such that the first and second shafts continue to drive withoutinterruption.

The invention can also be characterized as an improvement in a methodfor driving two shafts which are coupled by a flexible coupling. Theimprovement in the method comprises the steps of driving the shaftsthrough the flexible coupling as long as the flexible coupling does notfail. When the flexible coupling fails, the shafts are driven byengaging the plug means, which is coupled to one of the shafts, with asocket means, which is coupled to the other shaft. The plug and socketmeans are engaged with each other as a result of the failure of theflexible coupling. Again, by reason of the improvement in this method,flexibly coupled shafts will continue to drive without interruption evenwhen the flexible coupling connecting them fails.

These advantages and others derived from the present invention can bebetter understood by considering the following description of thepreferred embodiments in light of the figures wherein like elements arenumbered with like numerals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of a helicopter showing the environment inwhich a flexible coupling may be used to deliver power to a tail rotor.

FIG. 2 is a cross section of the coupling showing the insertion of theplug and socket means inside the flexible coupling.

FIG. 3 is an end view of the cross section shown in FIG. 2.

FIG. 4 is the end view of FIG. 3 after the coupling has failed showingthe fail-safe plug and socket combination engaged.

FIG. 5 is a cross section of the improved flexible coupling takenthrough section lines 5--5 of FIG. 3.

FIG. 6 is a perspective cutaway view of the socket and plug meansillustrating their relative shape and position.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is an improvement in a flexible coupling of thetype typically used in aircraft and in particular in rotary aircraftwhereby the delivery of power to a tail rotor may continue in anuninterrupted fashion even in the case where the flexible coupling,which connects shaft segments in the drive train, completely fails. Thefailure of the coupling actuates the engagement of the fail-safe meansincluded within the coupling so that power delivery continues withoutinterruption.

FIG. 1 illustrates the environment in which an improved coupling in thepresent invention may be used. Helicopter 10 is shown in phantomoutline, and includes a motor 12 coupled to a transmission 14 which inturn is coupled to a rotor hub 16 and blades 18. Transmission 14 alsoincludes a drive shaft 20 coupled to a rotor drive shaft 22 which isultimately coupled to tail rotor blades 24. Shafts 20 and 22 are coupledto each other through a flexible opening 26. Although coupling 26 hasbeen shown in the tail rotor drive transmission, it must be understoodthat flexible couplings are used throughout helicopter 10 in anyapplication where two shaft segments may be connected.

FIG. 2 is a cross section of that portion of the drive train indicatedby reference numeral 28 in FIG. 1. Shaft 20 is shown as coupled throughflexible coupling 26 to shaft 22. Flexible coupling 26 is well known tothe art and is sold by Bendix Corporation in a variety of models.

Flexible coupling 26 is particularly characterized as being comprised ofa first member 30 and a second member 32. First member 30 is securelycoupled to shaft 20 by conventional means and in particular in the priorart embodiment by a bolt whose head engaged member 30 and whose threadedbody is screwed into a mating, threaded hole provided in end 36 of shaft20. The means of coupling first member 30 to shaft 20 in the presentinvention is described below. Second member 32 is secured to shaft 22 byconventional means and is best shown in FIG. 3 as being three bolts 38coupled to a flange 40 in end 42 of shaft 22. In the particularembodiment illustrated, second member 32 has a triangular termination44, best shown in FIGS. 3 and 4.

Members 30 and 32 are flexibly coupled to each other by means of acircular diaphram 46 shown in FIG. 2 as being comprised of two thincircular steel sections 48 and 50 coupled respectively at their innerdiameters to members 30 and 32 and coupled together at theircircumferential edge by a bond 52. Thus, shaft 20 drives shaft 22although diaphram 46 which allows for a large degree of vibrationalfreedom between shafts 20 and 22 by virtue of the flexibility ofdiaphram plates 48 and 50. Although the present invention has beendescribed in connection with a specific type of flexible coupling itmust be understood that the invention is not so limited and could beused as an improvement in many other types of equivalent couplings bymaking modifications thereto which would be obvious to one with ordinaryskill in the art.

According to the improvement of the present invention, member 30 issecurely coupled to shaft 20 by means of a bolt 54 threaded in aconventional manner into hole 34 of shaft 20, but with a newlyconfigured head 56. Head 56 is comprised of a hex head tighteningportion 58 and a shoulder drive portion 60. This combination of elementsforms a plug means for securely coupling first member 30 to shaft 20. Aswe shall see, this plug means is arranged and configured to engage asocket means in the event that flexible coupling 26 should fail.

The socket means is comprised of an insert 62 which has a flange portion64 and a socket portion 66. Flange portion 64 forms a circular flangehaving a plurality of holes 68 machined in its circumferential area forcoupling with bolts 38. Bolts 38 are through-bolted through holes 68 asshown in FIG. 2 whereby the socket means, second member 32 and shaft 22form a rigidly coupled combination.

As best shown in FIGS. 3 and 4, socket portion 66 is particularlyadapted such that shoulder portion 60 of the plug means fits withinsocket portion 66 with sufficient clearance to allow the vibrationaldegrees of freedom normally expected between shafts 20 and 22. However,as particularly illustrated in FIG. 4 should flexible coupling 26 fail,shoulder portion 60 is driven against socket portion 66 such thatsurface 70 jams against surface 72 of socket portion 66. Surfaces 70 and72 are machined flat such that when shoulder portion 60 is drivenagainst socket portion 66, a flat surface to surface contact is madebetween surfaces 70 and 72. Thus, the engagement between the plug meansand socket means is solid and secure. No edges are knocked off, such aswould be the case of a hex head rotating within a hex socket. Thetorsional strength of the engagement between the plug means and socketmeans is so great that all other portions of the drive transmission arelikely to fail prior to the plug means or socket means shearing.

Hex head 58 does not engage a socket means in any manner and is solelyused for tightening bolt 54 within hole 34. Hex head 58 is recessed orhas a smaller effective diameter than shoulder portion 60 to allow foreasy and convenient insertion of a tool within the end of member 32during assembly. As before, shoulder portion 60 serves as a bolt head tosecure first member 30 to end 36 of shaft 20.

FIG. 5 better illustrates the arrangement and configuration of thesocket means and plug means within flexible coupling 26. FIG. 5 clearlyshows that the inside diameter of the socket means is not circular incross section but assumes a shape which closely circumscribes theoutline of shoulder portion 60. The amount of clearance is just largeenough to allow for all possible vibrational degrees of freedom aspreviously stated, yet is maintained small enough such that unacceptableimpact forces between shoulder portion 60 and socket portion 66 areavoided. When flexible coupling 26 fails, delivery of power betweenshafts 20 and 22 is interrupted for an insignificant amount of time,namely the short time required for shoulder portion 60 to engage socketportion 66. Inasmuch as shaft 22 is still rotating during that time, theimpact force between shoulder portion 60 and socket portion 66 isfurther reduced. In fact, the amount of disruption in the delivery ofpower between shafts 20 and 22 is so small that in most circumstancesthe pilot of the aircraft will be unaware that coupling 26 has failed.In most circumstances, the engagement and operation of the plug means incombination with the socket means will not create any perceptibledifference in the performance of the aircraft or audible noise. It isentirely possible and within the scope of the present invention thatvarious types of indicating devices may be incorporated in flexiblecoupling 26. However, improvement of the present invention does notrequire the use of such devices for its successful operation or use.

The degree of flexible coupling between shafts 20 and 22 is maintainedor slightly increased when the plug and socket combination of thepresent improvement is engaged. Surfaces 70 and 72 are maintained incontact solely by rotation of shafts 20 and 22. In other words, thesurfaces are pressed tightly together only by the frictional force andresistence of shaft 22 to the rotation of shaft 20. After the aircrafthas landed, there is no further resistive force transmitted from thetail rotor to shaft 22 to cause the tight engagement of surfaces 70 and72. At that time it becomes obvious that the tail rotor is much moreloosely coupled to the transmission than was previously the case. Thedegree of looseness is used as a reliable measure by ground crews thatflexible coupling 26 has failed and needs to be replaced before the nextflight.

FIG. 6 shows in a cutaway perspective the essential features of theimprovement constituting the present invention. It can be seen that thehex head portion 58 extends beyond the socket portion 66 and thatshoulder portion 60 is disposed within the socket hole defined by socketportion 66 with more than adequate clearance to allow for all degrees ofvibrational rotation during normal operation. The prespective view alsographically illustrates the simplicity and ruggedness of the combinationof the plug and socket that gives rise to the high reliability of thisfail-safe combination. The simplicity allows the improvement to beeconomically fabricated and results in a fault free performance.

Although the illustrated embodiment is a design which is intended to beretrofitted in existing flexible couplings, it should be clear that thebasic principle illustrated by the design can be incorporated in theoriginal design of couplings of all types with a minimum of increasedcomplexity and cost. Therefore, it must be understood that manymodification and alterations can be made to the present invention, whenapplied to originally manufactured couplings as well as a diversevariety of retrofitted couplings, without departing from the spirit andscope of the present invention as set forth in the following claims. Theillustrated embodiment has been discussed only as a means for clarifyingone application of the present invention and should not be taken as alimitation or restriction of the scope of the invention.

I claim:
 1. An improvement in a flexible coupling having a first membercoupled to a first shaft, and a second member coupled to a second shaftwith a flexible portion coupling said first and second members, saidimprovement comprising:plug means for coupling to said first shaft; andsocket means for coupling to said second shaft, said plug means arrangedand configured to loosely mate with said socket means in a firstcondition and to securely engage said socket means in a secondcondition; wherein said plug means has a screw portion and a headportion, said screw portion being threaded into said shaft and saidfirst member of said flexible coupling being secured to said first shaftby means of said head portion, said head portion having a shoulderportion and bolt head portion; whereby when said flexible coupling failssaid second condition obtains and said plug and socket means securelyengage each other such that said first and second shafts continue todrive.
 2. The improvement of claim 1 wherein said plug means has ashoulder portion in the form of two, oppositely disposed lobes, eachlobe being provided with at least one flat surface.
 3. The improvementof claim 2 wherein said socket means defines an opening having a shapeapproximating the shape of said shoulder portion and having a largeroutline so that said opening is everywhere displaced from said shoulderportion by a clearance when said socket means and plug means areconfigured in said first condition.
 4. The improvement of claim 3wherein said opening defined by said socket means has at least two flatsurfaces arranged and configured to contact two corresponding flatsurfaces provided on said shoulder portion of said plug means.